An AC induction motor is the most common type of motor used in industrial control systems. The AC induction motor offers simple, rugged construction, easy maintenance, and cost-effective pricing. Three-phase AC induction motors are utilized in many industrial environments, including chemical plants, foundries, pulp and paper plants, waste management facilities and rock crushers.
The basic components of an AC induction motor are the stator, rotor, and frame. The stator has coils of insulated wires, referred to as windings, which are directly connected to a power supply. The stator of a three-phase AC induction motor has three sets of windings. The stator windings are typically held stationary by the motor frame. Each stator winding is spaced at an equal distance from the other two windings and is connected to one of three lines of a three-phase power supply. The lines from the three-phase power supply provide current from each phase to the motor. As the windings magnetize in sequence, the phase currents also peak in sequence and create a rotating magnetic field within the stator. The rotating magnetic field produced by the stator windings produces a transformer-like effect, induces a current in the unpowered rotor windings, and causes the rotor to produce its own magnetic field. The magnetic interaction between the stator and the rotor magnetic fields is an attractive force resulting in rotor movement.
Despite its usefulness and wide-ranging application, the AC induction motor has certain associated limitations. One limitation, in particular, is that it is inherently incapable of providing a wide range of variable speeds in operation, when connected to a typical utility power supply. However, the operation of an induction motor at less than its rated speed is a desirable and useful feature in industrial applications. For example, in the mining industry, a three-phase AC induction motor is commonly used on a conveyer belt. In this application, the user may desire to have slow speed control over the motor for the inspection and repair of the belt. To satisfy this function it is necessary to slowly turn the motor at an exact speed to properly position the belt.
In order to operate AC induction motors at less than rated speed, various low speed techniques have been used in the past. In one method, the supply voltage to the motor is switched on for brief time periods to partially start the motor and then the motor is quickly disconnected from the source and the motor is allowed to coast. As a result, the motor slowly turns. Additional full voltage pulses at line input frequency are then applied to the motor windings intermittently to keep the motor turning. Power can be applied to the motor intermittently either through the use of an electromechanical contactor or through the use of a solid-state device such as a soft starter. The disadvantages of this method include high transient currents, high transient torques, and potential overheating of the motor. Furthermore, this method does not posses any inherent speed control because the voltage and frequency applied to the motor is not being altered other than being applied intermittently.
Another method of controlling the speed of an AC induction motor is through the use of a variable frequency drive. A variable frequency drive converts the supply voltage and frequency to another voltage and frequency so the induction motor can operate at less than the rated speed. Pulse width modulation, or PWM, drives are the most common type of variable frequency drive. The PWM drive contains electronic circuitry to convert AC line power to DC power. The PWM drive then pulses the DC output voltage for varying lengths of time to mimic a voltage output at the frequency desired. More specifically, the PWM drive produces a voltage waveform which, when applied to the motor, results in a motor current waveform that is essentially sinusoidal and of the frequency corresponding to the desired fundamental output frequency. By varying output voltage and frequency, a variable frequency drive controls the torque, speed, and direction of an AC induction motor. However, the variable frequency drive tends to be more complex and expensive in comparison to other low speed methods, especially as motor horsepower and motor rated voltage increases.
A solid-state reduced voltage starter, in addition to soft starting a motor, can be used to rotate an induction motor at less than rated speed. The solid-state starter is placed in series between the power supply and the motor and employs solid-state switches, such as Silicon Control Rectifiers or SCRs, to control the application of current flow and voltage to the motor. Each SCR in a soft-starter can be phase controlled or zero fired. Zero firing turns the SCR completely on so the voltage applied to the load is similar to that of an electromechanical switch or contactor.
Phase-control firing requires manipulation of the SCR firing angle. The firing angle is defined as the number of degrees from the beginning of the associated half-cycle of the AC waveform to the angle at which the gate voltage is applied. By controlling the firing angle, the soft starter is able to control the output voltage by turning the appropriate SCR or other switching device on for a particular portion of each half-cycle. When the SCR is turned on, or gated, voltage is applied to the load. The magnitude of the voltage applied to the load depends on the timing of the input power supply and when the SCR is gated on. Phase-control provides infinitely variable adjustability voltage between zero and full input voltage to the load as timed gate pulses are fed to each SCR. For example, the earlier in the half-cycle the SCR is gated on, the greater voltage is applied to the load.
Variables pertaining to the firing of the SCRs can be modified through the control electronics of the solid-state starter to increase or reduce the output voltage. The control electronics can be preprogrammed to provide a particular output voltage contour based on a timed sequence or the output voltage can be controlled based on measurements of current and/or motor speed.
Controlling the speed of a motor through the use of a solid-state starter has a number of advantages. As stated, the output voltage can be easily altered to suit the required load conditions. Furthermore, the SCRs are solid-state devices. Therefore, SCRs have no moving parts and provide high reliability and low maintenance operation compared to electromechanical motor controllers. Finally, the mechanical stress and shock on the motor is greatly reduced due to the reduction of large torque transients as a result of phase control providing quieter motor operation, longer equipment life, less maintenance and increased uptime.
Because of these advantages, the solid-state starter has found increasing utilization in connection with special SCR firing patterns to rotate a motor at slow speeds. One commonly used SCR firing pattern, known as a pulse skipping pattern, generates four pulses of current for each phase of each output cycle, two positive and two negative, to generate slow motor speed. When reduced speed is desired, the SCRs are controlled so that selected cycle portions from each phase of the power supply voltage are omitted from the voltage applied to the motor. Consequently, the fundamental frequency of the output voltage is a predetermined fraction of the fundamental frequency of the source voltage, and the running speed of the motor will be correspondingly reduced compared to full rated speed. Examples of solid-state starters utilizing a pulse skipping pattern include the Benshaw RediStart Microll product as well as soft starter products from Allen Bradley and SquareD.
One disadvantage of the pulse skipping pattern is that in order to achieve a given average current in the motor to produce a required level of torque with only four pulses of current per phase per output cycle, results in the four pulses having very high peak currents. The high peak currents can tax the input supply system, causing various disturbances such as light flicker. These high peak currents further cause additional heating of the motor and source transformer due to the high levels of current and related harmonic heating losses. In addition, the limitation of four pulses of current for each cycle constrains the overall average motor current that can be achieved in the motor. This reduction in motor current reduces the maximum torque that can be generated by the motor during cycle skipping slow speed operation. Cycle skipping also creates a cogging motion, which further creates mechanical harmonics on the shaft of the motor.
A particular example of a pulse skipping SCR firing pattern is disclosed in Rowan, et al., U.S. Pat. No. 4,996,470, issued for Feb. 26, 1991, for Electric Motor Speed Control Apparatus and Method, which discloses a device and method to reduce the speed of an induction motor. The speed of the motor is initially reduced through a combination of dynamic electrical braking and AC pulse skipping to a speed at which is it no longer synchronized to the AC input supply frequency. At such time, AC pulse skipping is employed to slow the AC motor down even further. The AC pulse skipping is continued for operation of the apparatus at a continuous speed until the point at which the user desires to reduce the motor speed to zero. Dynamic motor braking is again employed to break the motor out of synchronism with the pulse skipping frequency. The motor can then be slowed to a speed from which a very accurate final stoppage of the motor can occur to precisely position a work piece.
The previously described disadvantages of pulse skipping apply to Rowan, et al. In addition, Rowan, et al., the slow motor speeds are limited to only a few defined speeds. Typically, pulse skipping methods are limited to two discrete forward speeds of 7% and 14% of rated speed and two discrete reverse speeds of 10% and 20% of rated speed because of the timing involved with the incoming power supply lines. The speeds utilized in Rowan, et al., correspond to an effective frequency that is a fundamental frequency component of the AC power supply input line frequency. This effective frequency is defined in Rowan, et al. by the known expression:
      f    s        (                  6        ⁢        n            +      1        )  where n is an integer and f is the frequency of the supply voltage. Rowan, et al., further teaches that only fundamental frequency components of 1/7 and 1/13 are preferred because lower frequencies tend to drive the motor at too slow of a speed for many applications.
Another example of AC pulse skipping for speed control is represented by Asano, et al., U.S. Pat. No. 4,176,306, issued Nov. 27, 1979, for a Speed Control Apparatus. Asano, et al., discloses a speed control apparatus that includes a plurality of switches disposed between a three-phase AC power source and a motor in addition to a low speed control device for feeding power having a frequency lower than the frequency of the power source to the motor under the control of the switches.
Similar to Rowan, et al., Asano, et al., is limited by the effective frequencies of defined by the above equation. By limiting the effective frequency of the method, Asano, et al., have determined specific set of operating speeds, as described above.
What is lacking in the present art therefore, is a method of rotating a three-phase motor at speeds less than the rated synchronous speed while satisfying the following criteria: the speed of motor rotation should not be restricted to only a minimal set of speed selections; the available motor current and torque should be maximized the harmonic and flickering effects on the power supply should be minimized and motor heating should be minimized by utilizing the maximum number of current pulses per output cycle.